DESCRIPTION: A variety of cellular pathways for carbon and electron flow in the bacterium Escherichia coli and in other enteric bacteria are differentially employed depending on whether molecular oxygen is present in the cell environment. These include two distinct pathways for electron flow to oxygen via the cytochrome o and d oxidases encoded by the cyoABCDE and cydAB operons, respectively, or electron flow to any of a number of anaerobic electron acceptors. The alternative pathways for carbon flow include use of the TCA cycle reactions in a cyclic pathway during aerobic conditions, use of the branched and non-cyclic TCA pathway during aerobic and/or anaerobic conditions, and the mixed acid fermentation pathway used when no electron acceptors are present. This proposal addresses how the cell accomplishes the aerobic/anaerobic regulation of genes for the two cytochrome oxidase enzymes, and the genes of the TCA cycle pathway. The cell has two distinct regulators, comprised of the Fnr and the ArcA/ArcB proteins that participate in this task. Together, they coordinate gene transcription to help adjust carbon flow with electron flow in the cell for balanced and optimal growth. However, little is known about how the regulation of gene expression occurs at the molecular level. The major aims of this proposal are to understand how the ArcA and Fnr proteins participate in regulating cyoABCDE, cydAB, sdhCDAB, and icd gene expression. The binding sites of the proteins to DNA will be examined to understand how they function in repression or activation of transcription. The mechanism for microaerobic gene control of the cydAB genes will be studied, as will the coordinate regulation of the TCA cycle genes. Finally, the autoregulation of the arcA gene will be further examined since the ArcA protein is a major participant in the aerobic anaerobic switch in E. coli. These studies should provide a detailed understanding of how the cell coordinates pathways for electron and carbon flow within the cell during transition between oxygen rich and oxygen limiting conditions.